In the current electronic and/or computer system that is getting complicated and sophisticated, a power conversion device plays a very important role. Referring to FIG. 1 of the attached drawings, a control circuit diagram of a conventional power conversion device is shown. The conventional power conversion device, generally designated at 100, comprising a control logic circuit 1 that generates a series of pulse control signals Pout1 to control a switching circuit 2 to generate a series of switching circuit output signals LX1 by conducting on/off semiconductor devices (such as metal oxide semiconductor field effect transistors (MOSFETs) of the switching circuit 2 and then a filter circuit 21 is employed to convert an input voltage source Vin into an output voltage Vout1 for a load 3.
The control logic circuit 1 operates for output control with pulse width modulation (PWM). The PWM control features synchronization of a switching frequency of the switching circuit 2 with a system clock signal CLK and power conversion is realized through modulation of the pulse width of the pulse control signal Pout1. The modulation of the pulse width is based on feedback status signals of inductor current iL1 and output voltage Vout1. For example, an inductor current detection circuit 41, an error amplifier circuit 42, and a comparator 43 (that provides a slope compensation signal Vs) shown in the drawing constitute a feedback circuit of the output voltage Vout1. The inductor current detection circuit 41 functions to detect the inductor current iL1. The error amplifier circuit has an input terminal that is supplied with a reference voltage Vref and another input terminal that receives an input of the output voltage Vout1 supplied to the load 3. The comparator 43 compares outputs from the error amplifier circuit 42 and the inductor current detection circuit 41 and generates a feedback signal Fb that is fed back to the control logic circuit 1.
Besides PWM control, power conversion devices also adopt pulse frequency modulation (PFM) for power conversion. The PFM control features controlling switching frequency of a switching circuit and the switching frequency of the switching circuit is adjusted according to the difference of voltage conversion ratio or load. In case of high voltage conversion ratio and large load, the switching frequency of the switching circuit is increased, and in low voltage conversion ratio and small load, the switching frequency of the switching circuit is decreased. Compared to the PWM control, the PFM control maintains a fixed pulse width of the pulse control signal regardless of the voltage conversion ratio, the load, and variations of external devices, such as an inductor L and a capacitor C.
Generally speaking, for a power conversion device operated with PFM control, the efficiency of power conversion is high for light loading. When the lose of power for switching of the switching device is kept fixed, the ratio of the power lose of switching of the switching device with respect to output power is inversely proportional to the load. Thus, in light loading, a major portion of power lose occurs for the switching operation of the switching circuit. The PFM control reduces the switching frequency in light loading so that the power lose occurring for switching operation is reduced. Relatively speaking, the PFM control is of better efficiency in light loading than the PWM control, and the PWM control gets lowered conversion efficiency for light loading. Thus, the state-of-the-art power conversion device adopts an operation mode of combining PWM and PFM and switches from PWM control to PFM control in a light loading condition. FIGS. 2 and 3 respectively show waveforms of inductor current iL11, output voltage Vout11, and switching circuit output signal LX11 in a PWM mode and those of inductor current iL12, output voltage Vout12, and switching circuit output signal LX12 in a PFM mode.